Reflectance meters employing photoelectric cells to determine color values by measuring the amount of light reflected from a colored surface illuminated by a reference light source are well-known. These prior art reflectance meters have several disadvantages when it is sought to use them to quickly measure color values of analytical test devices, such as bibulous carriers impregnated with chemical reagent systems.
For some instruments, one of the disadvantages is the time and care required for making standardization adjustments. Repeated adjustments are required to compensate existing equipment for time and temperature variations. In particular, variations can occur using such equipment due to varying power to the light source. Power adjustments of prior art instruments generally corrected only the voltage and/or current to provide a substantially constant power input to the light source. Despite such adjustments error in the calibration of instruments could occur as a function of short term changes in light output from the light source or short term changes in sensitivity of the photoelectric cells. Repeated recalibration is not only an inconvenience, but it can introduce measurement errors if the recalibration is not accurately performed. Skilled personnel are accordingly required to operate the instruments.
Another major disadvantage of prior art instruments is the amount of heat generated by the light source and, concomitantly, the amount of power required for the light source. The utilization of heat filters to protect samples does not obviate this problem. Such heat filters do not minimize power requirements and they do reduce the intensity of the light source which can be effectively utilized. Substitution of shutter arrangements for heat filters to permit the utilization of full light intensity has also been suggested, but does not minimize the power requirements.
Prior art instruments also had a common drawback of being generally large in size and of substantial weight. Such instruments tended to be expensive due to their size, complexity and the necessity of adapting the equipment for regulation of heat generated during use. Some instruments were not even readily convertible for making both reflectance and transmittance measurements.
Still another problem with many existing reflectance meters is the fact that care must be exercised to regulate ambient light conditions during use in order not to affect the accuracy of the meters.